In recent years, separation membranes have been utilized for eliminating impurities in water as an alternative to conventional sand filtration or coagulating sedimentation process in the fields of water treatment involving beverage manufacturing, water purification treatment, waste water treatment and the like. Moreover, in the food engineering field, separation membranes have been utilized for the purpose of concentrating liquids, and separating and eliminating yeast etc., used in fermentation.
Since it is necessary that such separation membranes used in various ways can treat a large quantity of water in the water treatment fields, improvement of water permeability has been demanded. In addition, since a microbicide, an acid, an alkali, chlorine, a surfactant or the like is used for washing the separation membrane, chemical resistance properties have been required. Furthermore, in order to prevent contamination with any pathogenic microorganism or the like, the separation membranes are needed to have satisfactory separation characteristics and high physical strength such that the treated water is not contaminated with raw water. Thus, separation membranes are demanded to have excellent separation characteristics, chemical strength (chemical resistance), physical strength, contamination resistance, and permeability.
To meet such demands, separation membranes in which a polyvinylidene fluoride based resin is used, which has both chemical strength (chemical resistance) and physical strength have been employed (for example, Patent Document 1). However, the separation membranes in which a polyvinylidene fluoride based resin is used contain halogen molecules; therefore, there arises a problem of generating endocrine disrupting substances when the membranes are subjected to incineration disposal. Furthermore, since the polyvinylidene fluoride based resin is highly hydrophobic, there may also be a problem of rigid attachment of contaminating substances. Moreover, when a resin other than the polyvinylidene fluoride based resin, for example, a cellulose based resin is used, there arise problems of low chemical resistance and physical strength of the membrane.
In order to solve the problems of prior art as described above, production of a separation membrane using polyethersulfone that is superior in chemical resistance and mechanical strength has been investigated. However, a solvent applicable to polyethersulfone for use in a thermally induced phase separation method (TIPS method) capable of uniformly forming dense fine pores has not been known; therefore, film formation of polyethersulfone is not enabled unless a nonsolvent induced phase separation method (NIPS method) is performed according to the prior art.
However, the separation membrane of polyethersulfone produced by a nonsolvent induced separation method is accompanied by generation of macrovoids, and a uniform fine pore structure cannot be obtained, leading to inferior strength of the membrane. Therefore, various kinds of efforts are necessary for obtaining a practical applicable separation membrane, and thus the regulation method and quality control become difficult.
Patent Document 1: JP-A No. 2005-146230 (claim 4)